Control system for a fluid system

ABSTRACT

A control system for a variable displacement fluid pump or motor device having a displacement control mechanism operable upon actuation to vary the amount of fluid displaced by the device. An actuating means is operatively coupled to the displacement mechanism and actuates the same when communicated to a source of fluid pressure, the actuating means comprising a housing having a pressure chamber with a piston member movable therein in response to pressure fluid. The piston has a restricted passage which communicates pressure in the pressure chamber directly to a source of low pressure to provide a constant limited flow of control fluid throughout the control system prior to initial operation thereof to insure that the temperature of the fluid in the control system is elevated to a normal operating temperature.

nited States Patent Wk 14 1 Nov. 25, 1975 CONTROL SYSTEM FOR A FLUIDSYSTEM [76] Inventor: Philip A. Kubik, 6809 Spruce Drive, FreehBirmingham, Mich 48012 Asszstant Exammer-O. P. LaPomte Attorney, Agent,or Fzrm-Basrle and Wemtraub [22] Filed: June 22, 1973 [21] Appl. No.:372,696

57 ABSTRACT Related US. Application Data 1 [62] Division of Ser. No.236,736, March 21, 1972, A control system for a variable displacementfluid abandoned. pump or motor device having a displacement controlmechanism operable upon actuation to vary the CL 6; 60/ amount of fluiddisplaced by the device. An actuating [51] Int. Cl.2 FOIB 3/00 means isoperatively coupled to the displacement 1 Field of Search mechanism andactuates the same when communi- 417/2l7; 91/505, 506; 9 60/444 cated toa source of fluid pressure, the actuating means comprising a housinghaving a pressure cham- References Cited her with a piston membermovable therein in response UNITED STATES PATENTS to pressure fluid. Thepiston has a restricted passage 2,286,358 6/1942 Geiger 60/381 Whichcommunicates Pressure in the chamber 2197,33] 9/1942 wyligm 91/279directly to a source of low pressure to provide a con- 2,588,522 3/1952Harris 417/222 Stant limited flow of control fluid throughout the con-2,659,204 11/1953 Conway et al... 60/329 trol system prior to initialoperation thereof to insure ,447 10/1955 Hancock 60/431 that thetemperature of the fluid in the control system 3,676,020 7/1972Andreasen CI 31.. 91/506 i levated to a normal perating temperature3,700,356 10/1972 Kubik 417/222 FOREIGN PATENTS OR APPLICATIONS 1 Claim,4 Drawing Figures 41184 1970 Japan 417/217 US. Patent Nov. 25, 1975Sheet 1 of 2 3,921,503

Lid I I ./ZZ /f4 56 \M E M US. Patent Nov. 25, 1975 Sheet 2 of 2 CONTROLSYSTEM FOR A FLUID SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS Thisis a division of application Ser. No. 236,736, filed Mar. 21, 1972 nowabandoned.

The present patent application is related in substance to co-pending US.patent applications Ser. No. 50,093 filed June 26, 1970 now US Pat. No.3,653,208 and Ser. No. 67,177 filed Aug. 26, 1970 now US. Pat. No.3,700,356, both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a control system for controlling the displacementof a variable volume fluid device and in particular, the presentinvention relates to a system for elevating the temperature of the fluidin a control system prior to initiating a change in the displacement ofthe fluid device.

2. Description of the Prior Art Heretofore numerous fluid systems havebeen employed for controlling the rate of movement of a hydraulic pumpor motor device and, in particular, such fluid systems have foundextensive use in hydraulic machine tool transfer drives and the likesuch as the systems disclosed in the aforementioned co-pending US.patent applications. Such systems are used to accelerate and deceleratea fluid cylinder or rotary motor respectively at the beginning and theend of its stroke prior to the movement and generally consist of ahydraulic pump connected in either a closed-loop or open-loop fashion toa hydraulic motor, either a fluid cylinder or a rotary motor with fluidbeing delivered from the pump to the motor and returned from the motorto the pump or a reservoir. The rate of acceleration and deceleration ofthe fluid motor is controlled by varying the amount of fluid displacedby the pump which in turn is controlled by any suitable displacementcontrol mechanism. During the initial or start up phase of such systemsfluid in the control system is at a low temperature and thus the controlfluid has a high viscosity which results in the response time of thecontrol system during initial start up of the system to be such thatthedrive will not accelerate or decelerate in a manner which is sufficientto insure a smooth and quiet operation of the drive.

Thus it would be desirable to provide a means for elevating thetemperature of the fluid in the control system to a temperature levelwhich insures a proper operation of the control system during theinitial start up of the aforementioned drives. In the past, attempts tosolve this problem have included the provision of means for heating theoil in the control system, however, this method of insuring that theviscosity of the oil is maintained in a proper range to insure properoperation of the drives is either inadequate, cumbersome, or expensiveand, in addition, requires the operator of the machine to performadditional functions, all of which increases the expense of operatingand maintaining such previously used systems.

SUMMARY OF THE INVENTION The present invention which will be describedsubsequently in greater detail comprises in one embodiment a controlsystem for a fluid system including a source of 2 high pressure fluidsuch as a variable displacement pump, the output volume of which isadapted to be controlled by a displacement control mechanism. A fluidmotor device, such as a fluid cylinder, is operatively coupled to thedisplacement control mechanism of the variable displacement pump and isresponsive to a second source of fluid pressure to selectively move thedisplacement mechanism of the pump to increase or decrease the volume offluid displaced thereby. The control system includes a restrictedpassage which directly communicates the control fluid across the devicesuch that thereis a constant limited flow of fluid therethrough thecontrol system prior to initial start up operation of the control systemwhereby the temperature of the control fluid therein is elevated to anoperational temperature prior to initiating operation of the pump.

It is therefore an object of the present invention to provide a controlcircuit for a fluid system having means for elevating the temperature ofthe control fluid to an operating level prior to the commencement of theoperation of the fluid system.

Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art of control systems for fluidpump and motor devices when the accompanying description of severalexamples of the best modes contemplated for practicing the invention isread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The description herein makes referenceto the accompanying drawings in which like reference numerals refer tolike parts throughout several views, and in which:

FIG. 1 represents a schematic illustration of one example of a controlsystem constructed in accordance with the principles of the presentinvention;

FIG. 2 is a schematic illustration of a second example of a fluidcontrol system;

FIG. 3 is a schematic illustration of a third example of a fluid controlsystem; and

FIG. 4 is a schematic illustration of yet another example of a fluidcontrol system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand, in particular, to FIG. 1 wherein there is illustrated one exampleof the present invention in the form of a fluid control system 10comprising a fluid cylinder 12 operatively coupled to an external lever14 which, in turn, is connected to the displacement control mechanism ofa variable displacement device 16. The device 16 may be any suitablevariable displacement pump such as the pumps disclosed in theaforementioned United States patent applications. In the schematicillustrative example of FIG. 1 the pump 16 is considered for purposes ofexplanation to be at a minimum displacement or minimum flow positionwhen the lever 14 is rotated to the right and is in abutment with aminimum flow stop member 18 while the pump 16 is considered to be in amaximum flow position when the lever 1.4 is rotated to the left asviewed in FIG. 1 and is in abutment with a maximum flow stop member 20.The shifting of the lever 14 between the maximum and minimum flowpositions or any intermediate flow positions and the rate at which thelever 14 is so shifted controls the amount 'of fluid and the rate atwhich fluid is displaced by the pump 16 3 so as to control a fluid motoror the like such as specifically described in the aforementioned patentapplications. 1 t

The control system is but one example of a method of controlling thedisplacement of thevariable displacement pump 16 and comprises adirectional control valve 22 adapted to selectively connect fluid from asupply pump 24 to either of a pair of feed control valves 26 or 28. Thefeed control valves 26 and 28 are, in turn, respectively connected toports 30 and 32 of the fluid cylinder 12 by any suitable conduits or thelike. The fluid cylinder 12 is conventional in its construction andcomprises a-tubular housing 34 having an interior bore 36 divided intotwo pressure chambers 38 and 40 by means of a reciprocally mountedpiston 42 'which, in turn, carries on one side thereof a connecting rod44 that extends from the fluid cylinder 12 for coupling to'the pumplever 14. A conventional pressure relief valve 46 communicates with thesupply pump 24 and is adapted to limit the high pressure of the controlsystem 10 in a conventional manner.

The feed control valves 26 and 28 may be conventional in theirconstruction and have restricted passages 48and 50, respectively, whichare adjustable such that the feed control valves may be preset to supplyany desired flow rate over any desired range. Each of the feed controlvalves 26 and 28 further comprise check valves 52 and 54 which permitfluid to by-pass restricted'passages 48 and 50 in one direction of flow.Thus when the control valve 22 is in the position indicated in FIG. 1,

fluid flow is directed from the supply pump 24 through the feed valve 26viathe check valve 52 and is communicated to the'pressure chamber 38within the fluid cylinder 12 to exert a force on the piston 42 to shiftthe same rightwardly to displace the lever 14 toward a minimum flowposition, while at the same time the fluid in the pressure chamber 40.is exhausted from the fluid cylinder 12 through the'restricted passage.50 in the feed valve 28 and returned to a reservoir 56. When thedirection of the control 'valve 22 is reversed to direct fluid throughthe check valve 54 of the feed control valve 28, fluid is communicatedto the pressure chamber 40 whereupon the fluid pressure therein exerts aforce against the righthand side of the piston member 42 to shift sameleftwardly as viewed in FIG. 1 to position A and thereby shift the lever14 towards a maximum flow position while fluid in the chamber 38 isexhausted through the restricted passage 48 of the feed control valve 26and back to the reservoir 56 via direction control valve 22. 1

The piston 42 has a restricted orificeor passageway 58 which providesconstant fluid communicationbetween the pressure chambers 38 and 40. Theorifice 58 is so sized as to provide a limited amount of flow betweenthe two pressure chambers 38 and 40 and during normal operation asaforementioned the orifice 58 having negligible affect on fluid lossbetween the two pressure chambers. During the initial start up of asystem prior to actuation of the directional control valve 22 to causemovement of the piston 42 and thereby cause a change in the displacementof the pump 16, the orifice 58 provides a very simple and convenientmeans for elevating the temperature of the control fluid in the controlsystem 10 to eliminate aforementioned difficulties of a cold start up.

As can be seen in FIG. 1, if the cylinder is in the position illustratedand the directional control valve 22 is in the position illustrated;fluid from the pump 24, when 4 the same, is in operation, will bedirected to pressure chamber '38' exerting a force on the piston 42 andthereby retaining the pump lever l4 in the position illustrated, thatis," at a minimum flow position. During this mode of operation, fluidfrom the pressure chamber 38, which is a temperature in which theviscosity is relatively high and thus the fluid is difficult to moveresultingin a low response time for the system, will flow through theorifice 58 into the pressure chamber 40 and return to the reservoir inthe manner aforementioned. After several minutes of operation of thecontrol system'l0 in this manner, the temperature of the control fluidwill be elevated to its normal operating temperature and thus uponactuation of the directional control valve 22 to a positioncommunicating pressure fluid from the pump 24 to the pressure chamber 40to commence operation, the system fluid will be at a normal operatingtemperature and the aforementioned 1 thus a further detailed descriptionthereof is not necessary. The system 60 further comprises a'fluid pump62 of the axial piston type having as schematically illustrated arotating cylinder barrel 64 provided with aplurality of arcuately spacedcylinder bores 66 within which are reciprocally, mounted pistons 68 allof which is conventional and described in greater detail in theaforementioned US. patent application -Ser. No. 67,l77. The outer endsof the pistons 68 have shoes 70 which bear against a swash plate 72 thatis rotatable about an axis 74 to change the amount of stroking movementof the-pistons 68 within their respective bores 66 and thereby vary thedisplacement of the pump 62 and thus the volume of the fluid deliveredfrom the pump 62. The amount of inclination or the rotation of the swashplate 72 about the axis 74 is controlled-by a pair of diametricallyopposed piston members 76 and 78 whichare respectively slidably mounted1 in bores 77 and 79 in the housing of the pump 62. The

reciprocate within their respective bores 77 and 79.

The inner ends of the pistons 76 and 78 are respectively exposedto'pressure chambers 80 and 82 which when communicated to pressure fluidexert a force on their associated piston to move the same outwardly andthereby cause an inclination of the swash plate 72.

In, the embodiment illustrated in FIG. 2, fluid delivered from the feedcontrol valve 26 is communicated by any suitable conduit to the pressurechamber 82, while fluid delivered from the feed control valve 28 iscommunicated to the pressure chamber 80. It can thus be seen that duringnormal operation of the pump 62 when it is desired to bring the pump 62to a minimum displacement position, as illustrated, fluidis directedfrom the direction control valve 22 to the feed control valve 26 and tothe pressure chamber 82 wherein a force is exerted against the piston 78to move the same leftwardly and incline the swash plate 72 to a nearvertical position. At the same time fluid in the pressure chamber 80is'exhausted therefrom through the feed control valve 28 and returned toreservoir 56. Similarly when the direction control valve 22 is shifted,fluid is directed from the supply pump 24 through the feed control valve28 and to the pressure chamber 80 wherein a force is exerted against thepiston 76 to move the same outwardly and tilt the swash plate 72 aboutthe axis 74 while at the same time fluid within the pres sure chamber 82is exhausted therefrom through the feed control valve 26 and returned tothe reservoir 56.

Prior to the initial start up operation of the pump 62, the viscosity ofthe slug of oil in the pressure chambers 80 and 82 is high and thus theresponse time of the pump 62 is very slow, resulting in theaforementioned difficulties of a cold start up. In order to overcomethis disadvantage, the pressure chambers 80 and 82 are directlycommunicated by means of'a conduit 84 within the housing of the pump 62.The conduit 84 has re stricted orifices 86 which permit a limited flowbetween chambers 80 and 82 during the initial operation of the system 60but prior to shifting the pump swash plate 72 to cause either anincrease or decrease in displacement of the pump 62. The flow path isfrom the pump 24 into the pressure chamber 80 across the restrictedorifices 86 through conduit 84 into the pressure chamber '86 andexhausted therefrom for return to reservoir 56. The size of the orifices86 is such as to permit only a limited flow of fluid between thepressure chambers 80 and 82 and thus during normal operation of the pump62 in the manner described in the co-pending U.S. patent applicationSer. No. 67,177 there is a negligible loss of pressure and fluid betweenthe chambers 80 and 82 while at the same time during the initialoperation of the control system, but prior to a change in thedisplacement of pump 62 fluid will be communicated directly between thechambers 80 and 82 and circulate throughout the control system 60 bymeans of the supply pump 24 to provide a very simple means for elevatingthe temperature of the fluid in the control system 60 to operating leveland thereby eliminate the aforementioned cold start difficulties.

Referring now to FIG. 3, there is illustrated a control system 90 and avariable displacement pump 92 comprising a swash plate 94 adapted to berotated about a fixed axis 96 to vary the displacement of the pump 92and thus the volume of fluid delivered thereby. The pump 92 has acylinder barrel (not shown) in which pistons 98 are reciprocally mountedand having outer ends engaging the swash plate 94 in the conventionalmanner. Movement of the swash plate 94 about its axis 96 is accomplishedby means of a pair or diametrically opposed control pistons 100 and 102each having one end respectively disposed for reciprocal movement in thehousing of the pump 92 while the outer ends thereof engage the swashplate 94 such that when one of the pistons 100 or 102 is moved outwardlyand the other inwardly with respect to the pump-92, the swash plate 94is rotated about the axis 96. In the embodiment illustrated theeffective pressure responsive area of the control piston 100 isapproximately twice the effective pressure responsive area of thecontrol piston 102.

The control system 90 comprises a supply pump 104 which draws fluid froma reservoir 106 and constantly communicates pressure fluid to a pressurechamber 108 associated with the control piston 102 generating a force onthe piston 102 to urge the swash plate toward a maximum flow position.Fluid from the supply pump 104 is communicated by any suitable conduitthrough restricted passageway 110 of a feed control valve 112 to adirection of control valve 113 which is movable to a first position toblock the fllow of the fluid delivered from the supply pump 104 to asecond position (illustrated in FIG. 3) wherein fluid from the supplypump 104 is communicated to a check valve 114 of a second feed controlvalve 116 and to a pressure chamber 118 associated with the controlpiston 100. It can thus be seen that when the valve 113 is in theposition illustrated, fluid pressure is communicated to both pressurechambers 108 and 118 and the greater effective pressure responsive areaof the piston will cause the same to move outwardly to bring the swashplate 94 to a minimum full position while the control piston 102 movesinwardly into its associated pressure chamber 108 to exhaust fluidtherefrom.

If the direction of control valve 113 is shifted, the pressure chamber118 associated withthe control piston 100 is communicated via :anorifice 120 in the feed control valve 116 to the reservoir 106 whilefluid delivered from the supply pump 104 is communicated to only thepressure chamber 108 associated with the control piston 102 causing thesame to be shifted outwardly to rotate the swash plate 94 and bring thepump 92 towards a maximum flow condition. A pressure relief valve 122 isassociated with the control system 90 to limit the maximum value of thepressure therein in a conventional manner.

As is conventional in pumps of the axial piston type, that portion ofthe pump 62 in which the swash plate 94, the piston 98, and cylinderbarrel are disposed is normally referred to as a pump case 121 and isfilled with a fluid that is at a low pressure which may be at the inletpressure of the pump or some intermediate pressure such as the pressureof the fluid in the reservoir 106. As can best be seen in FIG. 3 thecontrol pistons 100 and 102 are respectively provided with restrictedpassages or orifices 122 and 124 which connect their associated pressurechambers 118 and 108 to the pump case 121 such that there is a constantflow of fluid from chambers 118 and 108 to the pump case 121. The sizeof the orifices 122 and 124 are such that the amount of flow passingfrom their associated chambers 108 and 118 will have no effect on theoperation of the pump 92 during normal operating modes. However, priorto the initial operation control system 90, that is, prior to actuatingeither of the control pistons 100 or 102 to cause a change in thedisplacement of the pump 92, the supply pump 104 can be activated toprovide flow to the pressure chamber 108 and to the pressure chambers118 whereupon fluid therein will flow through the restricted orifices122 and 124 to the pump case 121 and return to the reservoir 106 therebyresulting in a circulation of the fluid in the control system 90 causingan elevation of the temperature of the control fluid to a normaloperating level whereby the system is ready to commence operation whilehaving eliminated the aforementioned cold start problem.

Referring now to FIG. 4 where there is illustrated another example ofthe present invention comprising a fluid cylinder having an internalbore 132 within which is reciprocally mounted a piston 134 that dividesthe opposite portions of the bore 132 into pressure chambers 136 and138. The pressure chambers 136 and 138 are each connected by anysuitable conduit means to a reservoir 140 by means of a directionalcontrol valve 142. At the mid section of the fluid cylinder 130 there isprovided a pressure port 144 that communicates with a supply pump 146which, in turn, is

adapted to draw fluid from the reservoir 140 and deliver pressure fluidto the pressure port 144. The directional control valve 142 is movablefrom a first position wherein communication between the pressurechambers 136 and 138 and the reservoir 140 is open to a second positionwherein fluid communication between the reservoir 140 and the pressurechamber 136 is open while communication between the reservoir 140 andthe pressure chamber 138 is closed, to a third position whereincommunication between the reservoir 140 and the pressure chamber 138 isopened and communication between the pressure chamber 136 and thereservoir 140 is closed.

The piston 134 has extending from opposite sides thereof a pair ofconnecting arms that extend externally of the fluid cylinder 130 and areadapted to be operatively coupled to machinery or the like or whateveris desired to be driven by the fluid cylinder, for example the lever 14of the variable displacement pump 16 illustrated in FIG. 1. The piston134 is further provided with a restrictive T-shaped passage 148 whichcommunicates with the opposing pressure chambers 136 and 138 and whenthe piston 134 is disposed in the mid section of the fluid cylinder,passageway 148 communicates with the high pressure port 144.

When the control valve 142 is in the position which opens communicationsbetween the pressure chambers 136, 138 and the reservoir 140, fluidpressure from the pump 146 enters the T-shaped passageway 148 viapressure port 144 and communicates with both pressure chambers 136 and138 and the fluid is returned via valve 142 to the reservoir 140. Sincethe effective pressure response areas on both sides of the piston 134 isequal and the flow of fluid to both pressure chambers 136 and 138 is atthe same pressure, the piston 134 will not reciprocate within the fluidcylinder 130 during this mode. However, if, for example, the directionalcontrol valve 142 is positioned to close fluid communication between thepressure chamber 136 and reservoir 140 while the chamber 138communicates with the reservoir 140, then the pressure fluid flowinginto the chamber 136 from the T-shaped passageway 148 will exert a forceagainst the piston 134 to shift the same to the right thereby exhaustingfluid from the chamber 138 back to the reservoir 140. As the pistontraverses port 144, the same will open directly to the pressure chamber136 thereby allowing an increase in the rate of movement of the piston134. Similarly, if the pressure chamber 136 remains open to reservoir140 and communication between the reservoir and the pressure chamber 138is closed, the fluid communicated through the T-shaped passageway 140 tothe chamber 138 will increase in pressure therein resulting in a forcebeing exerted against the piston 134 to shift the same to the left asviewed in FIG. 4. Thus, it can be seen a simple means is provided forpermitting a circulation of fluid from the pump to both pressurechambers and back to the reservoir to maintain the fluid in the systemat an elevated temperature so that the response time of the system willnot be delayed during initial cold start up of the system.

The modification illustrated in FIG. 4 is particularly adapted to usewherein the cylinder is located at some remote distance from the valve142 and the conduits connecting the valve 142 to the pressure chambers136 and 138 is of a substantial length and exposed to a low temperaturesuch as when the same is employed in a cold temperature environment. Byhaving a constant circulation of fluid between the pressure chambers,the

pump, the reservoir and the valve, the aforementioned cold start upproblems are eliminated.

Thus, it can be seen the present invention provides a means forelevating the temperature of the fluid in a control system to anoperating level prior to the operation of the system and without havingto employ elaborate heating systems.

It can also be seen the same is accomplished by a very simplemodification to existing machinery not requiring new parts or a majorredesign of the components thereof.

While the forms of the embodiments of the present invention disclosedherein constitute preferred forms, it should be understood by thoseskilled in the art of fluid and control systems that other forms mightbe adopted all coming within the spirit of the invention and the scopeof the appended claims.

What is claimed is as follows:

1. A control system for a variable volume fluid pressure energytranslating device of a type having internal fluid displacement meansoperable upon actuation to vary the volume of fluid displaced by saiddevice, said control system comprising:

a source of high pressure fluid;

a source of low pressure fluid;

said fluid pressure energy translating device comprising:

a. a housing having inlet and outlet ports;

b. a cylinder barrel rotatably mounted within said housing, saidcylinder barrel having a plurality of arcuately spaced cylinder bores; aplurality of pistons with inner ends disposed for reciprocal strokingmovement within said cylinder bores;

0. means successively communicating said cylinder bores with said inletand outlet ports;

(1. an inclined swash plate mounted in said housing in a drivingrelationship with the outer ends of said pistons for imparting saidreciprocal stroking movement to said pistons within said cylinder barrelbores as said cylinder barrel rotates, the amount of fluid flowing fromsaid inlet port to said outlet port being a function of the amount ofinclination of said swash plate with respect to the axis of thereciprocal stroking movement of said pistons;

e. fluid displacement means operatively coupled to said swash plate tocontrol the amount of inclination of said swash plate and thus theamount of fluid volume displaced by said device, said swash plate beingrotatable about a predetermined axis, said fluid displacement meanscomprising: first piston slidably mounted in a first pressure chamber insaid housing and having an extended end with means engaging said swashplate, said first piston being adapted to extend under pressure fromsaid pressure chamber to rotate said swash plate about saidpredetermined axis in a first direction to increase the volume of fluiddisplaced by said device;

a second piston slidably mounted in a second pressure chamber in saidhousing said second piston having an extended end with means engagingsaid swash plate and adapted to extend under pressure from said secondpressure chamber to rotate said swash plate in a second direction todecrease the volume of fluid displaced by said device;

valve means for selectively communicating said source of high pressurefluid to one of said pressure chambers while exhausting the other ofsaid pressure chambers to said low pressure source to thereby controlthe amount of inclination of said swash plate and thus the amount offluid displaced by said device; and

restrictive passage means continuously connecting said pressurechambers, said restrictive passage means being so sized as to permit aflow of fluid from said high pressure source to said low pressure

1. A control system for a variable volume fluid pressure energytranslating device of a type having internal fluid displacement meansoperable upon actuation to vary the volume of fluid displaced by saiddevice, said control system comprising: a source of high pressure fluid;a source of low pressure fluid; said fluid pressure energy translatingdevice comprising: a. a housing having inlet and outlet ports; b. acylinder barrel rotatably mounted within said housing, said cylinderbarrel having a plurality of arcuately spaced cylinder bores; aplurality of pistons with inner ends disposed for reciprocal strokingmovement within said cylinder bores; c. means successively communicatingsaid cylinder bores with said inlet and outlet ports; d. an inclinedswash plate mounted in said housing in a driving relationship with theouter ends of said pistons for imparting said reciprocal strokingmovement to said pistons within said cylinder barrel bores as saidcylinder barrel rotates, the amount of fluid flowing from said inletport to said outlet port being a function of the amount of inclinationof said swash plate with respect to the axis of the reciprocal strokingmovemenT of said pistons; e. fluid displacement means operativelycoupled to said swash plate to control the amount of inclination of saidswash plate and thus the amount of fluid volume displaced by saiddevice, said swash plate being rotatable about a predetermined axis,said fluid displacement means comprising: a first piston slidablymounted in a first pressure chamber in said housing and having anextended end with means engaging said swash plate, said first pistonbeing adapted to extend under pressure from said pressure chamber torotate said swash plate about said predetermined axis in a firstdirection to increase the volume of fluid displaced by said device; asecond piston slidably mounted in a second pressure chamber in saidhousing said second piston having an extended end with means engagingsaid swash plate and adapted to extend under pressure from said secondpressure chamber to rotate said swash plate in a second direction todecrease the volume of fluid displaced by said device; valve means forselectively communicating said source of high pressure fluid to one ofsaid pressure chambers while exhausting the other of said pressurechambers to said low pressure source to thereby control the amount ofinclination of said swash plate and thus the amount of fluid displacedby said device; and restrictive passage means continuously connectingsaid pressure chambers, said restrictive passage means being so sized asto permit a flow of fluid from said high pressure source to said lowpressure source via said pressure chambers when said valve means isoperable to connect said high pressure source to said second pressurechamber whereby the temperature of said fluid may be initially elevatedwithout effecting said system said restrictive passage means being sosized as to have no effect on said system when said valve means isoperable to connect said high pressure source to said first chamber.